Method for correcting stray radiation in an x-ray computed tomography scanner

ABSTRACT

In a method for correcting for stray radiation in measured intensity values, the measured intensity values are obtained in an X-ray computed tomography scanner by means of a detector matrix that is situated in a tomography measuring field of the computer tomography scanner and has a multiplicity of detector elements arranged next to one another in a number of adjacent detector rows. At least one reference distribution of the stray radiation intensity is determined in the row direction of the detector matrix, and a stray radiation component of each measured value of intensity is determined starting from this at least one reference distribution, and the measured intensity values are corrected as a function of their respective stray radiation component. In this case, the stray radiation component of the measured values of intensity of at least a fraction of the detector rows is determined by using a recursion method on the basis of the reference distribution.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the correction of image artifacts inX-ray computed tomography that are caused by stray radiation.

2. Description of the Prior Art

Just like beam hardening effects scattering effects can cause undesiredimage artifacts in the reconstructed tomographic image of atransirradiated layer of a patient or some other object underexamination. These image artifacts simulate structures corresponding tono real anatomy of the object under examination, and therefore lead tomisinterpretations of the tomographic image. Particularly in the medicalsector, such misinterpretations can have grave consequences to theextent of endangering the life of the patient.

In order to suppress the stray radiation component in the radiationintensity values measured with a detector, it is known to collimate atthe detector side the X-ray radiation transirradiating the object underexamination. As a rule, collimators are produced from tungsten, which isvery well suited for this because of its high attenuation. However,tungsten has the disadvantage of being very expensive. This costdisadvantage is particularly significant when the detector is a detectormatrix with a multiplicity of detector elements arranged next to oneanother in a number of superimposed detector rows. In the case of suchdetectors, the depth of the collimator shaft provided for eachindividual detector element must be enlarged with an increasing numberof rows. The outlay for design and materials would be viewed as nolonger acceptable starting from a certain number of detector rows.

SUMMARY OF THE INVENTION

It is an object of the present invention to avoid with a lesser outlayimage artifacts caused by stray radiation in the case of multirowdetectors.

This object is achieved in a method for correcting stray radiation formeasured values of radiation intensity that are obtained in an X-raycomputed tomography scanner by means of a detector matrix that issituated in a tomography measuring field of the scanner and has amultiplicity of detector elements arranged next to one another in anumber of adjacent detector rows.

In accordance with the invention at least one reference distribution ofthe stray radiation intensity is determined in the row direction of thedetector matrix, and then a stray radiation component of each measuredintensity value is determined starting from this reference distribution,and the measured intensity values are corrected as a function of theirrespective stray radiation component. The stray radiation component ofthe measured intensity values of at least a fraction of the detectorrows are determined by recursion in the following way:

-   -   a) the stray radiation component of the measured intensity        values of a current detector row of the recursion is determined        from the measured intensity values of this current detector row        and a primary radiation component of the measured intensity        values of a preceding detector row of the recursion,    -   b) the primary radiation component of the measured intensity        values of the preceding detector row is determined from the        measured intensity values of this preceding detector row and the        stray radiation component thereof, and    -   c) intensity values from the reference distribution of the stray        radiation intensity are used as stray radiation component of the        measured intensity values of a first detector row of the        recursion.

As used herein primary radiation means that radiation component of thetotal radiation incident on the detector elements that reaches thedetector matrix without being scattered, that is to say on a direct pathfrom the radiation source of the computer tomography scanner. Atomography measuring field means a measuring zone fitted with detectorelements in which the measured total radiation includes a primaryradiation component. As a rule, the tomography measuring field is fixedby a diaphragm arrangement at the source side.

In the solution according to the invention, the stray radiationcomponent is estimated by calculation for all detector rows with usingat least one reference distribution. Expensive collimator shafts thuscan be dispensed with. The recursion, which is applied at least for afraction of the detector rows, offers the basis for taking account of aprofile of the stray radiation component that changes across the span ofthe detector rows.

In a first refinement of the method according to the invention, the atleast one reference distribution of the stray radiation intensity isobtained from measured values of the reference intensity that areobtained by measuring radiation intensity outside the tomographymeasuring field. The fact that no primary radiation occurs outside thetomography measured field is thereby utilized. Measuring elementsarranged there consequently detect only stray radiation. It is easilypossible to determine a distribution of the stray radiation in the rowdirection, which is then used as the reference distribution.

The radiation intensity will expediently be measured above a firstdetector row of the detector matrix or/and below a last detector row ofthe detector matrix.

In general, the spatial profile of the stray radiation can berepresented by a comparatively low-frequency function. It is thereforesufficient to record measured values for the stray radiation in the rowdirection only in a relatively coarse array. In other words, themeasured values of reference intensity are preferably obtained atmeasuring points that are situated at a mutual spacing in the rowdirection of the detector matrix and of which the number is smaller, inparticular much smaller than the number of the detector elements perdetector row. The reference distribution of the stray radiationintensity then can be easily obtained by interpolation of the measuredvalues of reference intensity.

It is even possible to obtain one reference distribution by measuringthe radiation intensity above the first detector row of the detectormatrix, and a further reference distribution by radiation intensitymeasurement below the last detector row of the detector matrix.

The recursion preferably should be begun at least in a detector row onthe edge of the detector matrix. The assumption that the stray radiationintensity outside the tomography measuring field differs—if at all—onlyinsubstantially from the stray radiation intensity in a detector row atthe edge will apply here, as a rule. Consequently, the error that ariseswhen intensity values from the reference distribution are used as strayradiation component of the measured values of intensity from thedetector row at the edge will be negligible.

In a second refinement of the method according to the invention, the atleast one reference distribution of the stray radiation intensity iscalculated by using the measured intensity values of at least onedetector row of the detector matrix. In particular, it is possible inthis case for the reference distribution to be calculated on the basisof a mathematical convolutional model. Such a convolutional model isknown, for example, for a computed tomography scanner with detectorelements arranged in a single row from B. Ohnesorge: “Untersuchungen derScatter-Korrektur in Elektronenstrahl-Computertomographen Chair ofInformation Technology of the University of Erlangen-Nuremberg,Dissertation 1994. By adapting this convolutional model to a multirowdetector matrix, it is possible to estimate the stray radiationdistribution for a detector row of the matrix by calculation from themeasured intensity values obtained for this detector row.

It could be noted that the stray radiation distribution couldfundamentally be calculated in each case in all detector rows with theaid of the above convolutional model, and that a recursion would then besuperfluous. However, convolutional operations can be very demandingcomputationally. The application of recursion for at least a fraction ofthe detector rows renders it possible, by contrast, to keep thecomputational outlay within acceptable limits and, at the same time, totake account of possible changes in the stray radiation distributionfrom detector row to detector row.

The reference distribution will expediently be calculated by using themeasured intensity values of a middle detector row of the detectormatrix, and the recursion will be begun toward upper and lower detectorrows at least in this middle detector row. It goes without saying,however, that the reference distribution can also be calculated with theaid of the measured values of intensity of another detector row, inparticular even of a detector row at the edge.

In order to improve the quality of the results obtained for the strayradiation component of the measured intensity values, the recursion canbe ended after a fraction of detector rows, and a further recursion canbe started in a subsequent detector row. The further recursion can bestarted in this case on the basis of the same or another referencedistribution of the stray radiation intensity.

If the object under examination includes comparatively contrastingstructures, the measured intensity values can change relatively stronglyfrom detector row to detector row and/or within a detector row fromdetector element to detector element. This is due to a rapid change inthe stray radiation (which—as already mentioned—changes onlycomparatively slowly in space, as a rule) but is due to spatiallychanging attenuation properties of the transirradiated material. So thatsuch instabilities in the measured total intensity do not substantiallyfalsify the stray radiation components, which are used in the finalanalysis to correct the measured values of intensity, the strayradiation components determined after carrying out the recursionpreferably are subjected to low pass filtering in the column directionand, if desired, also in the row direction of the detector matrix. Thelow pass filtering filters out from the recursively determined strayradiation components those changes of intensity, which have acomparatively high spatial frequency. These are usually a result ofchanges in the attenuation properties. The filtered stray radiationcomponents thus reproduce the low frequency profile of the strayradiation very well. The measured intensity values are then corrected asa function of their respective filtered stray radiation component.

A refinement of the estimate obtained for the stray radiation componentof the measured intensity values is possible when starting from twodifferent reference distributions, two values of the stray radiationcomponent are determined for each measured intensity values, and themeasured intensity values are corrected in accordance with a respectiveaveraged stray radiation component.

Independently of the recursive determination of the stray radiationcomponent, the inventive method includes determining the referencedistribution by means of measuring the radiation intensity outside thetomography measuring field. According to a second aspect, the inventiontherefore further provides a method for correcting stray radiation formeasured values of radiation intensity that are obtained in an X-raycomputed tomography scanner by means of a detector matrix that issituated in a tomography measuring field of the computed tomographyscanner and has a multiplicity of detector elements arranged next to oneanother in a number of adjacent detector rows. In this case according tothe invention that at least one reference distribution of the strayradiation intensity in the row direction of the detector matrix isobtained from measured values of reference intensity that are obtainedby measuring radiation intensity outside the tomography measuring field,and that a stray radiation component of each measured intensity value isthen determined starting from this at least one reference distribution,and the measured intensity values are corrected as a function of theirrespective stray radiation component.

In order to estimate the stray radiation component of the measuredintensity values, it is also possible to apply the recursion, explainedearlier, with the steps a) to c). However, it is also conceivable to usean intensity value from the reference distribution of the strayradiation intensity as the stray radiation component. In this case, thereference distribution is simply taken over directly as stray radiationdistribution for each detector row. In cases where the stray radiationintensity actually changes only slightly over the span of the detectormatrix, it is possible already to achieve very good results. If, bycontrast, marked changes in the stray radiation intensity must be dealtwith, preference will be given to the recursive mode of procedure.

Of course the method according to the second aspect can be configured bymeans of further features of the method according to the first aspect.

Additionally the invention is directed to an X-ray computed tomographyscanner, which is designed for carrying out the method according to thefirst or/and second aspect. In particular, in the computed tomographyscanner it is possible to provide an auxiliary detector arrangement,arranged outside the tomography measuring field, for obtaining themeasured values of reference intensity. Above a first detector row ofthe detector matrix or/and below a last detector row of the detectormatrix the auxiliary detector arrangement can have a number of auxiliarydetector elements that are arranged at mutual spacings in the rowdirection of the detector matrix and of which each supplies one of themeasured values of reference intensity. The number of the auxiliarydetector elements in the row direction of the detector matrix is in thiscase preferably smaller, in particular substantially smaller, than thenumber of the detector elements per detector row.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an embodiment of a CT scanner according tothe invention with a multirow detector matrix.

FIG. 2 is a schematic plan view of the detector matrix as seen in thedirection of the arrow II in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The CT scanner shown in the figures has an X-ray source 10 and adetector arrangement 12. The X-ray source 10 emits X-ray radiation inthe shape of a fan, as indicated at 14. An object under examination 16arranged in the beam path between the X-ray source 10 and the detectorarrangement 12 is penetrated by the X-ray radiation. The detectorarrangement 12 detects the X-ray radiation downstream of the objectunder examination 16. Specifically, the detector arrangement 12 has adetector matrix 18 composed of a multiplicity of detector elements 20that are distributed over a number of adjacent rows and are arrangednext to one another in each row in the direction of a fan angle β. Foursuch detector rows are shown as an example in FIG. 2; however, thenumber of the detector rows can differ therefrom as desired and can be8, 16 or 24, instead, for example. The size of the beam fan 14 in thedirection of the fan angle β can be set by means of a diaphragmarrangement 22 that is arranged between the X-ray source 10 and theobject under examination 16. The radiation emitted by the X-ray source10 is likewise bounded by a comparable diaphragm arrangement (not shown)in the column direction of the detector matrix 18, that is to say in adirection z in FIG. 2. In the region of the detector arrangement 12, thediaphragm arrangement 22 and the z-diaphragm arrangement just addresseddefine a tomography measuring field within which it is possible todetect the primary radiation that strikes the detector arrangement 12 ona straight path from the X-ray source 10 without being scattered in theobject under examination 16. The detector matrix 18 is situatedcompletely within this tomography measuring field. Each position in thedirection of the fan angle β at which a detector element 20 is locatedcorresponds to a projection channel.

Each detector element 20 detects the radiation incident in its zone ofspace and supplies a corresponding intensity measuring signal I_(G)(n,k) to an electronic evaluation and reconstruction unit 24. Here, theindex n stands for the number of the row of the detector matrix 18 inwhich the relevant detector element 20 is located, while k representsthe channel number. The evaluation and reconstruction unit 24 firstlycarries out a stray radiation correction on the incoming intensitymeasuring signals I_(G)(n, k) by subtracting a stray radiation componentI_(S)(n, k) from the intensity measuring signals I_(G)(n, k). Thisleaves a primary radiation component I_(P)(n, k) that is representativeof the intensity of the primary radiation incident on the respectivedetector element 20. The evaluation and reconstruction unit 24 thendetermines attenuation values from the intensity values I_(P)(n, k) thatit uses to reconstruct a tomographic image, displayed on a monitor 26,of the transirradiated layer of the object under examination 16. The CTscanner requires projections from a multiplicity of different directionsin order to reconstruct the tomographic image. The X-ray source 10 canbe moved for this purpose in the direction of the arrow 28 around theobject under examination 16.

In order to be able to carry out the stray radiation correction, the CTscanner is designed to start by determining a reference distribution ofthe stray radiation intensity in the row direction. This referencedistribution specifies, for each channel k, a reference valueI_(Sref)(k) for the stray radiation intensity. In order to determine thereference distribution, the detector arrangement 12 has in addition tothe detector matrix 18, a number of auxiliary detector elements 30 (seeFIG. 2). These are situated outside the tomography measuring field andtherefore are not struck by primary radiation but exclusively by strayradiation. The auxiliary detector elements 30 consequently permitmeasured information to be obtained on the intensity of the strayradiation. The auxiliary detector elements 30 are also connected to theevaluation and reconstruction unit 24 and supply their measuring signalsto the same.

The auxiliary detector elements 30 are arranged above the uppermost rowin the z-direction, and/or below the lowermost row in the z-direction,of the detector matrix 18. Since the spatial distribution of the strayradiation can be described in general by a comparatively low frequencyfunction, a coarse array of the auxiliary detector elements 30 sufficesin the row direction, and so by comparison with the number of detectorelements 20 present per row only a substantially smaller number, forexample smaller by an order of magnitude, of auxiliary detector elements30 is preferably provided in the row direction. The evaluation andreconstruction unit 24 then uses interpolation to determine thereference distribution I_(Sref)(k) supplied by the auxiliary detectorelements 30. The auxiliary detector elements 30 are expedientlydistributed at uniform spacings in the row direction; this is not,however, mandatory. Of course, it is not excluded to provide a number ofauxiliary detector elements 30 in the row direction that is equal to thenumber of the detector elements 20.

In channel k (k=1, . . . , N) for the total intensity I_(G)(n, k)measured in the detector row n (n=1, . . . , L):I _(G)(n, k)=I _(P)(n, k)+I _(S)(n, k)  (1)

The aim of the stray radiation correction carried out in the evaluationand reconstruction unit 24 is firstly to estimate the stray radiationcomponent I_(S)(n, k) as accurately as possible in order subsequently tohave available values for the primary radiation component I_(P)(n, k)that are as accurate as possible and can be fed to the imagereconstruction.

Estimation of the stray radiation begins in the uppermost or thelowermost detector row depending on whether the reference distributionI_(Sref)(k) was obtained from the measuring signals of auxiliarydetector elements 30 situated above or below the detector matrix 18. Itis assumed below that the operation begins in the uppermost detectorrow. The channel number is no longer specified explicitly in this case,in order to simplify the notation. The following considerations apply,however, for any desired angular positions in the beam fan, and thus forany desired channel numbers. For the uppermost detector row:I _(G)(1)=I _(P)(1)+I _(S)(1)  (2)

It is assumed for the purpose of determining the primary radiationintensity I_(P)(1) in the uppermost (first) detector row that I_(Sref)and the stray radiation component I_(S)(1) of the first detector rowdiffer from one another—if at all—only negligibly. The primary radiationintensity I_(P)(1) therefore can be calculated in a simple way asfollows:I _(P)(1)=I _(G)(1)−I _(Sref)  (3)

The primary radiation intensities in all further detector rows can nowbe determined similarly by assuming that the primary radiation intensityI_(P)(n−1) of the n−1th detector row corresponds approximately to theprimary radiation intensity I_(P)(n) of the nth row. With thisassumption, the stray radiation intensity I_(S)(n) in the nth row can becalculated recursively as follows from the actually measured totalintensity I_(G)(n) in this row and the primary radiation intensityI_(P)(n−1) in the preceding row n−1:I _(S)(n)=I _(G)(n)−I _(P)(n−1)  (4)

The primary radiation intensity I_(P)(n) of the nth row then can beestimated in accordance with:I _(P)(n)=I _(G)(n)−I _(S)(n)  (5)

The assumption I_(P)(n−1)=I_(P)(n) is justified in general in the caseof low-contrast structures. If, however, the object under investigation16 includes contrasting structures such as bone, for example,significant changes can occur in the measured total intensity betweenconsecutive rows and/or channels. The estimated values I_(S)(n) of thestray radiation intensity are subjected to low pass filtering ofselectable length, for example with a median filter. This means thatsignal instabilities upon transition from row n−1 to row n in the aboverecursion are not carried over to the calculation of the stray radiationintensities and therefore falsify the I_(P)(n) values. The low passfiltering removes the instabilities discussed above. The filteredI_(S)(n) values then reflect a very good estimate of the actual strayradiation intensity. Subsequently, new I_(P)(n) values that are used forimage reconstruction are calculated from the filtered I_(S)(n) values bysubstitution in the above equation (5).

The low pass filtering can be carried out as one-dimensional filteringin the z-direction, or else as two-dimensional filtering in the z- androw directions.

If auxiliary detector elements 30 are provided above and below thedetector matrix 18, two reference distributions I_(Sref),1 andI_(Sref),2 can be determined, specifically one (I_(Sref),1) from themeasuring signals of the auxiliary detector elements 30 situated abovethe detector matrix 18, and the other (I_(Sref),2) from the measuringsignals of the auxiliary detector elements 30 situated below thedetector matrix 18. The above method for recursive estimation of theprimary radiation intensities can then be carried out twice,specifically once beginning in the uppermost detector row on the basisof the reference distribution I_(Sref),1, and once beginning in thelowermost detector row on the basis of the reference distributionI_(Sref),2. Thus, two values I_(P),1 and I_(P),2 of the primaryradiation intensity that are subsequently averaged are obtained for eachdetector element 20. The averaged intensity values are then used for theimage reconstruction.

In some instances, it can already suffice to use the referencedistribution I_(Sref) obtained with the aid of the auxiliary detectorelements 30 as a model for the stray radiation distribution of all thedetector rows of the detector matrix 18. The primary radiationintensities I_(P)(n) can then easily be calculated as follows:I _(P)(n)=I _(G)(n)−I _(Sref)  (6)

It is also conceivable not to continue the recursion of all the detectorrows, but to truncate it after a fraction of the detector rows, forexample, after each second, third or fourth detector row or after halfof the detector rows, and then to start a new recursion in a newdetector row. In this new detector row, the assumption is made again, ina way similar to equation (3), that the stray radiation distribution ofthis row corresponds to the reference distribution I_(Sref). It is evenpossible to conceive of proceeding from a different referencedistribution upon restarting the recursion. In the above example withauxiliary detector elements 30 above and below the detector matrix, itcould be sensible, for example, to carry out a recursion for the upperhalf of the detector rows on the basis of the reference distributionI_(Sref),1 and to carry out a recursion on the basis of the referencedistribution I_(Sref),2 for the lower half of the detector rows, inparticular when the detector matrix 18 has a large number of rows, forexample 16, 24 or 32.

The reference distribution I_(Sref) can also be determined in anotherway than with the aid of the auxiliary detector elements 30. Thus, forexample, it is possible to compute the associated stray radiationdistribution I_(S)(n) of a row of the detector matrix 18 from themeasured values of intensity I_(G)(n) of said row. This stray radiationdistribution I_(S)(n) can then be used as reference distributionI_(Sref) in order to estimate for the remaining rows of the detectormatrix 18 the stray radiation component of the measured values ofintensity of these rows by means of the above recursion method.

A convolutional model for a single-row detector system is known from theliterature, cited above, of B. Ohnesorge for the purpose of computing astray radiation distribution from measured values of intensity. Thismodel is based on the idea, in principle, that the functionaldependences of the scattering angle on the differential active crosssections and scattering energies of Compton and Raleigh scatteringjustify the assumption that the scattering contributions in a detectorchannel k that belongs to a fan angle β_(k) decrease with the angularspacing in the fan (β−β_(k)). (Only simple scattering processes aretaken into account in the derivation.) A “spacing function” G(β) thatcan be used for the description then has a maximum at β=β_(k) and sweepsover the angular range (−β_(max)+β_(k), β_(max)+β_(k)). A strayradiation distribution I_(SC)(β) dependent on the fan angle β is thenyielded as follows:I _(SC)(β)=C _(m) ·f(Δz _(sl))·(I _(SC,forw)(β){circle around(×)}G(β))·R(β)  (7)

Here, C_(M) denotes a machine constant and f(Δz_(sl)) a weightingdependent on layer thickness. I_(SC,forw)(β) is a forward strayradiation intensity, calculated in the model of single scattering, with:$\begin{matrix}{I_{{SC},{forw}}\left( {{\beta\beta} = {K_{{SC},{forw}} \cdot {I\left( {\beta\left( {{\cdot \left( {- \ln} \right)}\left( \frac{I\left( {\beta(} \right.}{I_{0}} \right)} \right)} \right.}}} \right.} & (8)\end{matrix}$

K_(SC,forw) is a proportionality constant, I₀ the intensity of thenon-attenuated radiation, and I(β) the radiation intensity measured inthe fan angle β of the detector system. The scattering contributions ofall the beams in the fan to all the detector elements are taken intoaccount in the convolutional equation (7) by the convolutional kernelG(β); $\begin{matrix}{{G(\beta)} = \left( {1 + \left( \frac{\beta}{A \cdot {\Delta\beta}} \right)^{2}} \right)^{- k}} & (9)\end{matrix}$is usually specified as spacing kernel. A is parameter with which thewidth can be controlled. It can be determined empirically from imageoptimizations or from a comparison of stray radiation distributionscalculated in the convolutional model and simulated ones. For thefunction R(β)R(β)=1, if βε[=β_(max), β_(max)]; 0 otherwise  (10)

Further information on the above convolutional model for single-rowdetector systems can be taken from the literature of B. Ohnesorge.

This known model can now be modified within the scope of the inventionin order to adapt to multirow or two-dimensional detectors such asshown, for example, in FIG. 2. Equation (7) can easily be expandedbecause of the rotational symmetry of the differential active crosssections with regard to the fan coordinate β and the row coordinatez_(n) (z_(n)=(L/2−n)Δz; (n=1, . . . ,L)). (Δz represents the rowheight). The stray radiation intensity I_(SC)(β, z_(n)) is then givenby:I _(SC)(β,z _(n))=C _(M) ·f(βz _(sl))·(I _(SC,forw)(β,z _(n)){circlearound (×)}G(β,z _(n)))·R(β,z _(n))  (11)

In this case, C_(M) and f(Δz_(sl)) have the same meaning as above.I_(SC,forw)(β, z_(n)) is, in turn, the forward scattering radiationintensity calculated in the model of single scattering, with$\begin{matrix}{{I_{{SC},{forw}}\left( {\beta,z_{n}} \right)} = {K_{{SC},{forw}} \cdot {I\left( {\beta,z_{n}} \right)} \cdot \left( {{- 1}{n\left( \frac{I\left( {\beta,z_{n}} \right)}{I_{0}} \right)}} \right)}} & (12)\end{matrix}$

I(β, z_(n)) denotes the radiation intensity measured in the fan angle βof the nth detector row. For R(β, z_(n)) that:R(β,z _(n))=1, if βε[−β_(max), β_(max)] and 1≦n≦L; 0 otherwise  (13)The spacing kernel is now: $\begin{matrix}{{G\left( {\beta,z_{n}} \right)} = \left( {1 + \left( \frac{\beta^{2} + \left( \frac{z_{n}}{R_{fd}} \right)^{2}}{A^{\prime} \cdot {\Delta\beta}} \right)} \right)^{- k}} & (14)\end{matrix}$

Here, A′ in turn denotes the width parameter, β²+(z_(n)/R_(fd))²measures the distance from the detector origin to the detector elementin the fan angle β of the nth detector row, and R_(fd) denotes thespacing between the focus and detector of the CT scanner.

The stray radiation distribution I_(SC)(β, z_(n)) can be calculated inthe previous way for an arbitrary row of the detector matrix 18. It isrecommended to calculate it for a middle detector row. The strayradiation distribution thus calculated is then used as referencedistribution I_(Sref) for the recursion. The recursion is started in therow from whose measured intensity values the reference distribution wascalculated. In the case of a middle detector row, both a recursion toupper detector rows and recursion to lower detector rows are started. Ifthe recursion is interrupted after a fraction of detector rows, a newrecursion is begun in a new row, preferably with a new referencedistribution that was calculated with the aid of the above convolutionalmodel from the measured values of intensity of this new row. A highquality can be achieved in this way in estimating the accurate strayradiation components.

Although modifications and changes may be suggested by those skilled inthe art, it is the invention of the inventor to embody within the patentwarranted heron all changes and modifications as reasonably and properlycome within the scope of his contribution to the art.

1. A method for correcting for stray radiation in an X-ray computedtomography scanner comprising the steps of: irradiating a detectorarrangement, composed of a multiplicity of detector elements arrangednext to each other in a plurality of adjacent detector rows, with X-raysin a tomography measuring field of a computed tomography scanner; fromeach of said detector elements, obtaining a measured intensity valuerepresenting an intensity of X-rays incident thereon; obtaining at leastone reference distribution of stray radiation intensity in a rowdirection of said detector arrangement and, for each of said measuredintensity values, determining a stray radiation component thereofdependent on said at least one reference distribution; and for at leastsome of said detector rows, correcting the respective measured intensityvalues of the detector elements thereof as a function of the respectivestray radiation components of those measured intensity values byrecursion, including the steps of: determining the respective strayradiation components of the measured intensity values of the detectorelements in a current row in said recursion from the respective measuredintensity values of the detector elements of said current detector row,and respective primary radiation components of the respective measuredintensity values of detector elements of a preceding detector row insaid recursion, determining said respective primary radiation componentsof the measured intensity values of the detector elements of saidpreceding detector row from measured intensity values of said detectorelements of said preceding detector row and the respective strayradiation components of said measured intensity values of the detectorelements of said preceding detector row, and using intensity values fromsaid reference distribution as the respective stray radiation componentsof the measured intensity values of detector elements of a firstdetector row in said recursion.
 2. A method as claimed in claim 1wherein the step of determining said at least one reference distributioncomprises: disposing a plurality of auxiliary detector elements of saiddetector arrangement outside of said tomography measuring field; andobtaining measured intensity values from said auxiliary detectorelements for producing said at least one reference distribution.
 3. Amethod as claimed in claim 2 wherein said detector arrangement comprisesa detector matrix having a first detector row and a last detector row,and wherein the step of disposing a plurality of auxiliary detectorelements outside of said tomography measuring field comprises disposingsaid auxiliary detector elements at at least one location selected fromthe group consisting of above said first detector row and below saidlast detector row.
 4. A method as claimed in claim 3 wherein each ofsaid rows of said detector matrix consists of a number of said detectorelements, and wherein the step of disposing a plurality of auxiliarydetector elements outside of said tomography measuring field comprisesdisposing a plurality, which is less than said number, of said auxiliarydetector elements at equal spacings in said row direction outside ofsaid tomography measuring field, and obtaining said referencedistribution by interpolating the respective measured intensity valuesfrom said plurality of auxiliary detector elements.
 5. A method asclaimed in claim 3 comprising beginning said recursion for respectivemeasured intensity values of detector elements of a detector row of saiddetector matrix selected from the group consisting of said firstdetector row and said last detector row.
 6. A method as claimed in claim2 wherein said detector arrangement comprises a detector matrix having afirst detector row and a last detector row, and wherein the step ofdetermining said reference distribution comprises the steps of:disposing a first plurality of auxiliary detector elements above saidfirst detector row and disposing a second plurality of auxiliarydetector elements below said last detector row; and obtaining a firstreference distribution from respective measured intensity values fromsaid first plurality of auxiliary detector elements and obtaining asecond reference distribution from respective measured intensity valuesfrom said second plurality of auxiliary detector elements.
 7. A methodas claimed in claim 1 wherein the step of determining said at least onereference distribution comprises calculating said at least one referencedistribution from respective measured intensity values of detectorelements in at least one detector row of said detector arrangement.
 8. Amethod as claimed in claim 7 comprising calculating said at least onereference distribution using a mathematical convolution model.
 9. Amethod as claimed in claim 7 wherein said detector arrangement has amiddle detector row, and wherein the step of calculating said referencedistribution comprises calculating said reference distribution usingrespective measured intensity values from the detector elements of saidmiddle detector row, and beginning said recursion for detector rowsmoving outwardly in said detector arrangement from said middle detectorrow.
 10. A method as claimed in claim 1 comprising ending said recursionafter proceeding through each detector row in said portion of detectorrows, and then beginning a further recursion starting with a subsequentdetector row not in said first portion.
 11. A method as claimed in claim10 comprising starting said further recursion using a same referencedistribution as was used for said recursion.
 12. A method as claimed inclaim 10 comprising starting said further recursion using a furtherreference distribution, different from said reference distribution usedfor said recursion.
 13. A method as claimed in claim 1 wherein saiddetector arrangement has a column direction perpendicular to said rowdirection, and comprising the additional steps of: low-pass filteringsaid stray radiation components determined from said recursion in atleast one direction selected from the group consisting of said columndirection and said row direction, to obtain respective filtered strayradiation components for said measured intensity values; and correctingeach of said measured intensity values as a function of the filteredstray radiation component thereof.
 14. A method as claimed in claim 13comprising employing a median filter for said low-pass filtering.
 15. Amethod as claimed in claim 1 wherein the step of obtaining at least onereference distribution comprises obtaining two different referencedistributions of said stray radiation intensity for each of saidmeasured intensity values, and correcting each of said measuredintensity values with an averaged stray radiation component obtainedfrom said two different reference distributions.
 16. A method forcorrecting for stray radiation in an X-ray computed tomography scannercomprising the steps of: irradiating a detector arrangement, composed ofa multiplicity of detector elements arranged next to each other in aplurality of adjacent detector rows, with X-rays in and outside of atomography measuring field of a computed tomography scanner; from eachof said detector elements, obtaining a measured intensity valuerepresenting an intensity of X-rays incident thereon; obtaining at leastone reference distribution of stray radiation intensity in a rowdirection of said detector arrangement from said measured intensityvalues from said detector elements outside of said tomography measuringfield and, for each of said measured intensity values, determining astray radiation component thereof dependent on said at least onereference distribution; and for at least some of said detector rows insaid tomography measuring field, correcting the respective measuredintensity values of the detector elements thereof as a function of therespective stray radiation components of those measured intensityvalues.
 17. A method as claimed in claim 16 comprising, for eachmeasured intensity value, using an intensity value from said referencedistribution as said stray radiation component.
 18. A method as claimedin claim 17 wherein said detector arrangement comprises a detectormatrix having a first detector row and a last detector row, andcomprising the step of disposing a plurality of auxiliary detectorelements outside of said tomography measuring field at at least onelocation selected from the group consisting of above said first detectorrow and below said last detector row.
 19. A method as claimed in claim18 wherein each of said rows of said detector matrix consists of anumber of said detector elements, and wherein the step of disposing aplurality of auxiliary detector elements outside of said tomographymeasuring field comprises disposing a plurality, which is less than saidnumber, of said auxiliary detector elements at equal spacings in saidrow direction outside of said tomography measuring field, and obtainingsaid reference distribution by interpolating the respective measuredintensity values from said plurality of auxiliary detector elements. 20.A method as claimed in claim 16 wherein said detector arrangementcomprises a detector matrix has a first detector row and a last detectorrow, and wherein the step of determining said reference distributioncomprises the steps of: disposing a first plurality of auxiliarydetector elements above said first detector row and disposing a secondplurality of auxiliary detector elements below said last detector row;and obtaining a first reference distribution from respective measuredintensity values from said first plurality of auxiliary detectorelements and obtaining a second reference distribution from respectivemeasured intensity values from said second plurality of auxiliarydetector elements.
 21. A method as claimed in claim 16 comprisingdetermining said stray radiation component of the respective measuredintensity values from detector elements in at least a portion of saiddetector rows by recursion, including the steps of: determining therespective stray radiation components of the measured intensity valuesof the detector elements in a current row in said recursion from therespective measured intensity values of the detector elements of saidcurrent detector row, and respective primary radiation components of therespective measured intensity values of detector elements of a precedingdetector row in said recursion, determining said respective primaryradiation components of the measured intensity values of the detectorelements of said preceding detector row from measured intensity valuesof said detector elements of said preceding detector row and therespective stray radiation components of said measured intensity valuesof the detector elements of said preceding detector row, and usingintensity values from said reference distribution as the respectivestray radiation components of the measured intensity values of detectorelements of a first detector row in said recursion.
 22. A method asclaimed in claim 21 wherein said detector arrangement comprises adetector matrix having a first row and a last row, and comprisingbeginning said recursion for respective measured intensity values ofdetector elements of a detector row of said detector matrix selectedfrom the group consisting of said first detector row and said lastdetector row.
 23. A method as claimed in claim 21 comprising ending saidrecursion after proceeding through each detector row in said portion ofdetector rows, and then beginning a further recursion starting with asubsequent detector row not in said first portion.
 24. A method asclaimed in claim 21 comprising starting said further recursion using asame reference distribution as was used for said recursion.
 25. A methodas claimed in claim 24 comprising starting said further recursion usinga further reference distribution, different from said referencedistribution used for said recursion.
 26. A method as claimed in claim21 wherein said detector arrangement comprises a detector matrix havinga column direction perpendicular to said row direction, and comprisingthe additional steps of: low-pass filtering said stray radiationcomponents determined from said recursion in at least one directionselected from the group consisting of said column direction and said rowdirection, to obtain respective filtered stray radiation components forsaid measured intensity values; and correcting each of said measuredintensity values as a function of the filtered stray radiation componentthereof.
 27. A method as claimed in claim 26 comprising employing amedian filter for said low-pass filtering.
 28. A method as claimed inclaim 16 wherein the step of obtaining at least one referencedistribution comprises obtaining two different reference distributionsof said stray radiation intensity for each of said measured intensityvalues from said detector elements outside of said tomography measuringfield, and correcting each of said measured intensity values from saiddetector elements in said tomography measuring field with an averagedstray radiation component obtained from said two different referencedistributions.
 29. An X-ray computed tomography scanner comprising: adetector arrangement including a detector matrix, composed of amultiplicity of detector elements arranged next to each other in aplurality of adjacent detector rows; an X-ray source for irradiatingsaid detector arrangement, with X-rays in a tomography measuring field;said detector arrangement from each of said detector elements,generating a measured intensity value representing an intensity ofX-rays incident thereon; an evaluation and reconstruction unit fordetermining at least one reference distribution of stray radiationintensity in a row direction of said detector matrix and, for each ofsaid measured intensity values, determining a stray radiation componentthereof dependent on said at least one reference distribution; and saidevaluation and reconstruction unit configured to correct for at leastsome of said detector rows, correcting the respective measured intensityvalues of the detector elements thereof as a function of the respectivestray radiation components of those measured intensity values byrecursion, by determining the respective stray radiation components ofthe measured intensity values of the detector elements in a current rowin said recursion from the respective measured intensity values of thedetector elements of said current detector row, and respective primaryradiation components of the respective measured intensity values ofdetector elements of a preceding detector row in said recursion,determining said respective primary radiation components of the measuredintensity values of the detector elements of said preceding detector rowfrom measured intensity values of said detector elements of saidpreceding detector row and the respective stray radiation components ofsaid measured intensity values of the detector elements of saidpreceding detector row, and using intensity values from said referencedistribution as the respective stray radiation components of themeasured intensity values of detector elements of a first detector rowin said recursion.
 30. A computed tomography scanner as claimed in claim29 wherein said detector arrangement includes a plurality of auxiliarydetector elements disposed outside of said tomography measuring field,and wherein said evaluation and reconstruction unit determines saidreference distribution from respective measured intensity values fromsaid auxiliary detector elements.
 31. A computed tomography scanner asclaimed in claim 30 wherein said detector matrix has a first detectorrow and a last detector row, and wherein said auxiliary detectorelements are disposed at equidistant spacings in said row direction atat least one location selected from the group consisting of above saidfirst detector row and below said last detector row.
 32. A computedtomography scanner as claimed in claim 31 wherein each row of saiddetector matrix consists of a number of said detector elements, andwherein said plurality of auxiliary detector elements is smaller thansaid number.
 33. A computed tomography scanner as claimed in claim 32wherein each of said rows of detector elements in said detector matrixcomprises a number of detector elements, and wherein said detectorelements outside of said tomography measuring field comprise a pluralitywhich is less than said number.
 34. An X-ray computed tomography scannercomprising: a detector arrangement, composed of a multiplicity ofdetector elements arranged next to each other in a plurality of adjacentdetector rows; an X-ray source for irradiating said detector arrangementwith X-rays in a tomography measuring field, said detector arrangementincluding detector elements in said tomography measuring field anddetector elements outside of said tomography measuring field; saiddetector arrangement, from each of said detector elements, generating ameasured intensity value representing an intensity of X-rays incidentthereon; an evaluation and reconstruction unit for determining at leastone reference distribution of stray radiation intensity in a rowdirection of said detector arrangement from said measured intensityvalues for said detector elements outside of said tomography measuringfield and, for each of said measured intensity values from said detectorelements in said tomography measuring filed, determining a strayradiation component thereof dependent on said at least one referencedistribution; and for at least some of said detector rows in saidtomography measuring field, correcting the respective measured intensityvalues of the detector elements thereof as a function of the respectivestray radiation components.
 35. A computed tomography scanner as claimedin claim 34 wherein said detector arrangement includes a detector matrixhaving a first detector row and a last detector row, and wherein saiddetector elements disposed outside of said tomography measuring fieldare disposed at equidistant spacings in said row direction at at leastone location selected from the group consisting of above said firstdetector row and below said last detector row.